There have been known materials for rotor shafts, on which a magnetic field winding is mounted, such steels as shown in JP-B-47-25248 and JP-B-50-7530. In particular, as a generator shaft material necessitating not lower than 700 MPa of tensile strength at room temperature, a steel containing not less than 3.0% nickel and not more than 2.0% chromium as disclosed in ASTM Standard A469 has generally been used.